1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a printer and the like, and more particularly it relates to an image heating apparatus for heating an image on a recording material.
2. Related Background Art
In the past, in many image forming apparatuses of electrophotographic type such as electrophotographic copying machines, electrophotographic printers and the like, as fixing means, a fixing device of contact heating type having a heat roller and having good thermal efficiency and good safety or a fixing device of film heating type having reduced energy consumption has been used.
The fixing device of heat roller fixing type mainly comprises a fixing roller (heat roller) as a heating rotary member, and an elastic pressure roller as a pressure rotary member urged against the fixing roller and is designed so that, while a recording material (transfer sheet, electrostatic recording paper, electrofax paper, printing paper or the like) on which a non-fixed image (toner image) was formed and born is being passed through a fixing nip (abut nip portion) between the rotating rollers, the non-fixed image is permanently fixed to the recording material by heat from the fixing roller and pressure in the fixing nip.
Further, the fixing device of film heating type is disclosed in Japanese Patent Application Laid-open Nos. 63-313182, 2-157878, 4-44075, 4-44076, 4-44077, 4-44078, 4-44079, 4-44080, 4-44081, 4-44082, 4-44083, 4-204980, 4-204981, 4-204982, 4-204983 and 4-204984, for example, and comprises a heating body (heater) and a heat-resistive fixing film (heating rotary member) slidingly contacted with the heating body by a pressurizing rotary member (elastic roller) and is designed so that, while a recording material on which a non-fixed image was born is being passed together with the fixing film through a fixing nip portion between the heating body and the pressurizing rotary member with the interposition of the fixing film, the non-fixed image is permanently fixed to the recording material by heat from the heating body via the fixing film and pressure in the fixing nip portion.
In the fixing device of film heating type, since a wire-shaped heating body having low heat capacity can be used as the heating body and a thin film having low heat capacity can be used as the fixing film, electric power can be saves, and weight and time reduction (on-demand, quick start) can be achieved.
As the wire-shaped heating body having low heat capacity, a so-called ceramic heater can generally be used. The ceramic heater mainly comprises a ceramic substrate made of alumina, aluminum nitride or the like, and a heating body provided on the substrate and capable of generating heat by energization.
Thus, the fixing device of film heating type has various advantages such as unnecessity of waiting pre-heating and elimination of a waiting time due to high heating efficiency and fast rising-up. Particularly, since a method in which a cylindrical film is driven by a conveying force of a pressurizing roller can be realized with low cost, such a method has been adopted to low speed compact image forming apparatuses and is expected to be introduced into large-sized high speed image forming apparatuses in the future.
In the fixing device of film heating type, it is required that a length of a heating element of the heating body be equal to or greater than a maximum of a sheet size, and temperature control of the heating body is effected by detecting a temperature of the heating body by means of a thermistor (temperature detecting element) disposed in the vicinity of a longitudinal center of the heating body. Thus, when a sheet having maximum size is passed, the heat generated from the heating body is absorbed by the sheet, with the result that the temperature of the entire heating body is decreased.
On the other hand, when a sheet having a size smaller than the maximum size is passed in a center standard, since the temperature of only a central portion of the heating body on which the sheet is passed is decreased, the temperature of non-sheet passing portions of the heating body is increased in comparison with the central portion (non-sheet passing portion temperature increase phenomenon), with the result that portions of the film and the pressurizing roller corresponding to both lateral edge portions of the heating body may be damaged. Further, after the small sized sheet was passed, if a larger sized sheet is passed, offset (adhesion of toner to the film) will occur by the influence of the temperature-increased edge portions.
Conventionally, in order to solve this problem, a method in which through-put is reduced (i.e., print frequency is reduced) to widen sheet passing interval has been utilized.
However, the non-sheet passing portion temperature increase phenomenon has become more severe due to high speed tendency of the on-demand fixing device of film heating type, and, thus, it is very difficult to solve the above problem only by reduction of through-put.
In order to solve such a problem, it is considered that heating elements having different lengths and widths are provided on a substrate of the heating body so that the heating elements are selectively energized in accordance with a sheet size of a recording material to be passed (zone heating).
FIGS. 10A and 10B shows an example of the zone heating which is background of the present invention. In FIG. 10A, there are provided a ceramic heater 100 as a heating body, a heater holder 2, a heat-resistive fixing film 3 and an elastic pressurizing roller 4.
The heater 100 is held by the heater holder 2 with a heating surface facing downwardly, and the elastic pressurizing roller 4 is urged against the downwardly facing heating surface of the heater 100 with the interposition of the fixing film 3, thereby forming a fixing nip portion N.
The heater 100 is heated and temperature-adjusted to a predetermined temperature by energization of heating elements. The fixing film 3 is slid on the downwardly facing heating surface of the heater 100 in the fixing nip portion N and is shifted in a direction shown by the arrow.
In a condition that the heater 100 is heated and temperature-adjusted to the predetermined temperature and the fixing film 3 is shifted in the direction shown by the arrow, when a recording material P on which a non-fixed toner image t was formed and born is introduced between the fixing film 3 and the elastic pressurizing roller 4 at the fixing nip portion N, the recording material P is conveyed together with the fixing film 3 through the fixing nip portion N while being closely contacted with the surface of the fixing film 3. In the fixing nip portion N, the recording material P and the toner image t are heated by the heater 100 via the fixing film 3, with the result that the toner image t on the recording material P is thermally fixed to the recording material P. A portion of the recording material passed through the fixing nip portion N is separated from the surface of the fixing film and is conveyed.
FIG. 10B is a partially sectioned schematic plan view of the ceramic heater 100 as the heating body (showing a back side of the heater). The heater 100 comprises a heater substrate 100a having a longitudinal direction perpendicular to a sheet passing direction, two parallel heat generating member for large sized sheet (large sized sheet heating element) h1 and heat generating member for small sized sheet (small sized sheet heating element) h2 formed on the back surface of the heater substrate 100a along the longitudinal direction thereof, electricity supplying electrode pattern portions a, b, c for the heating elements h1, h2, and a glass coating layer 100b over-coated on the heating element forming surface of the heater substrate.
The heater substrate 100a is a ceramic substrate having insulation capacity, good heat transferring ability and low heat capacity and is made of aluminum nitride in this example.
The heating elements h1, h2 are heat generating resistance bodies for generating heat by energization and are formed by pattern-printing and firing heat generating resistance paste made of silver palladium (Ag/Pd), Ta.sub.2 N or the like.
The electricity supplying electrode pattern portions a, b, c are formed by pattern-printing and firing silver (Ag) paste.
The glass coating layer 100b is formed by pattern-printing and firing glass paste. The glass coating layer 100b is provided for protecting the heating elements h1, h2 and for ensuring insulation against an electric element such as a thermistor and the surface of the film.
In this example, in the heater 100, a surface of the heater opposite to the surface on which the heating elements h1, h2 are formed is used as a heating surface on which the fixing film 3 is closely contacted and slidingly shifted (back surface heating heater).
The large sized sheet heating element h1 corresponds to the maximum sheet passing width for LTR size (width=215.9 mm), A4 size (width=210 mm), EXE size (width=184.2 mm) and C5 size (width=162 mm) and has a length L1 of 222 mm.
The small sized sheet heating element h2 is provided for envelopes of DL size (width=114 mm), com10 (=104.7 mm) and monarch (=98.4 mm) and has a length L2 of 116 mm.
In this example, the sheet is passed with center standard (reference).
Among the electricity supplying electrode pattern portions a, b, c, the electricity supplying electrode pattern portion c serves as a common electrode for the heating elements h1 and h2.
When the large sized sheet is passed, the large sized sheet heating element h1 is used for heating, and, when the small sized sheet is passed, the small sized sheet heating element h2 is sued for heating. In this way, the zone heating is effected. That is to say, when the large sized sheet is passed, electricity is supplied between the electricity supplying electrode pattern portions a and c to cause the large sized sheet heating element h1 to generate heat, thereby coping with the passing of the large sized sheet. When the small sized sheet is passed, electricity is supplied between the electricity supplying electrode pattern portions b and c to cause the small sized sheet heating element h2 to generate heat, thereby coping with the passing of the small sized sheet. In this way, the non-sheet passing portion temperature increase can be prevented.
Although not shown, a thermistor (temperature detecting element) is provided to be contacted with the surface of the glass coating layer 100b on the back surface of the heater in the vicinity of the longitudinal center of the heater. The temperature of the heater is detected by the thermistor and a temperature adjusting circuit so that the temperature of the heater can be controlled.
Although it is impossible to correspond lengths of heating elements to all of sheet sizes, as shown in FIGS. 10A and 10B, even when two kinds of heating elements h1, h2 having different lengths to cope with the main sheet sizes are provided, increase in temperature of the non-sheet passing portions can be suppressed, thereby greatly improving a print speed for the small sized sheet.
However, the above-mentioned zone heating has the following disadvantage.
That is to say, as mentioned above, the large sized sheet heating element h1 having larger length and the small sized sheet heating element h2 having smaller length are arranged side by side on the heater substrate 100a and a sheet width sensor is disposed inside of the small sized sheet heating element h2 with respect to the longitudinal direction.
The sheet width sensor recognizes the passed recording material as a large sized sheet when it detects the passed recording material and recognizes the passed recording material as a small sized sheet when it does not detect the passed recording material.
The reason why the sheet width sensor is disposed inside of the small sized sheet heating element h2 is that, regarding the temperature distribution of the pressurizing roller, since there is temperature sagging (reduction) at portions of the roller corresponding to ends of the heating elements, if the sheet width sensor is arranged outside of the small sized sheet heating element, poor fixing may occur at both lateral edges of the small sized sheet, and, thus, the sheet width sensor must be disposed inside of the small sized sheet heating element.
By the way, although the length of the small sized sheet heating element h2 is set to be greater than the maximum width of the small sized sheet in consideration of the end temperature reduction, since the heating element h2 is essentially provided for coping with the non-sheet passing portion temperature increase, it is preferable that the length of the heating element is made small as less as possible. So long as the sheet width sensor is disposed outside of the maximum width of the small sized sheet and inside of the minimum width of the large sized sheet, there is no problem. However, since the sheet width sensor is disposed inside of the small sized sheet heating element h2 the length of which is made small as less as possible, a distance between the end of the maximum width of the small sized sheet and the position of the sheet width sensor becomes very small. Thus, even when the sheet width sensor is disposed inside of the small sized sheet heating element h2, if the recording material is deviated laterally away from the sheet width sensor, the lateral edge of the recording material is substantially aligned with the end of the heating element h2, with the result that poor fixing may occur.
Further, if the small sized recording material is skew-fed, it is passed outside of the sheet width sensor, with the result that the sheet width sensor recognizes the recording material as a small sized sheet, thereby energizing the large sized sheet heating element h1. In this case, portions of the pressurizing roller and the film corresponding to the non-sheet passing portions may be damaged or offset due to end temperature increase may occur.